The present invention relates to a mass rate of flow meter of the angular momentum type having a swirl generator for imparting angular momentum to the measured fluid, a rotor that is rotated by the fluid stream, and a restrained reaction turbine for removing the imparted angular momentum. More particularly, the invention relates to a flowmeter of the above-mentioned type having improved rotor and turbine constructions for improving the performance of a flowmeter over a wide range of temperatures.
A mass rate of flow meter of the angular momentum type relating to the present invention is disclosed by Hildebrand et al in U.S. Pat. No. 4,056,976, issued Nov. 8, 1976, titled "Mass Rate of Flow Meter", and assigned to the same assignee as the present invention. This patent shows a mass flowmeter in which a swirl generator imparts angular momentum to the fluid to be measured. Immediately downstream from the swirl generator is an unrestrained rotor. Swirling fluid from the swirl generator is directed through passageways in the rotor provided by multiple cylindrical tubes, causing the unrestrained rotor to rotate at the average velocity of the fluid stream. The restrained reaction turbine is similar to the rotor, having passageways formed by multiple tubes through which fluid from the rotor flows. In this flowmeter, the angular momentum of the fluid stream angularly displaces the turbine until the bias of a restraining spring balances the torque on the turbine, allowing measurement of flow rate by means not material to the present invention and not described here.
When such flowmeters are used to measure the flow of fuel in an aircraft, both the fuel and the flowmeter are subjected to a wide range of operating temperatures. The effect on the fuel is manifested, in part, by a change in fuel viscosity which, in turn, can lead to inaccurate indications of flow measurements. More specifically, it appears that the momentum imparted to the rotor and leakage through the flowmeter past the rotor and turbine become dependent upon viscosity, and, therefore temperature.
Referring first to the changes in momentum, prior flowmeters utilize plural, cylindrical tubes to form the rotor passageways. Such construction is shown in U.S. Pat. No. 2,148,150 by Cornell, issued Aug. 9, 1960, and titled "Mass Flowmeter Baffle"; this patent is assigned to the same assignee as the present invention. Given a constant viscosity, the pattern of fluid flow, or flow profile, becomes nearly constant over a wide range of flow rates and establishes a radius of gyration. Thus, it is possible to calibrate the flowmeter because the radius of gyration remains nearly constant. However, when the viscosity changes, the flow profile and the radius of gyration also change, and these changes manifest themselves as errors in the readings.
Now referring to leakage through the flowmeter, it is desirable that the rotor and turbine move with as little friction as possible. This allows the angular speed of the rotor to be solely a function of the angular velocity of the fuel leaving the rotor. It also allows the displacement of the turbine to be determined by the restraining spring. Thus, the outer peripheries of rotor and turbine must not contact the inner surfaces of the housing. Moreover, the need to minimize friction restricts the possible sealing methods that might be used to prevent leakage, especially at low flow rates. To date flowmeters are constructed with a small, annular clearance between the housing and each of the rotor and turbine. Some leakage does occur through this clearance, but in the past this has been compensated during calibration.
In current applications of these flowmeters, this leakage can introduce inaccuracies into flow indications. As the viscosity of the fuel changes, the percentage of fuel leaking through the clearance also can change in an unpredictable manner. Moreover, at low fuel flow rates, the fuel that passes through the clearance, and therefore is not measured, can become a significant percentage of the total fuel passing through the flowmeter. These changes in leakage are not easily compensated and so it is possible that inaccurate readings can be obtained.